1. Field of the Invention
The present invention relates generally to retrieval of magnetically stored data using a magnetoresistive (MR) head, and more particularly, to a circuit and method for providing a true voltage bias to an MR head.
2. Related Art
A Magnetoresistive (MR) head is commonly used to retrieve data stored on a magnetic medium, such as a disk drive. Data is read by detecting variations in the resistance of a sensor stripe of the MR head caused by variations in the magnetic field emanating from the storage medium. This resistance is represented by R.sub.MR. R.sub.MR consists of a constant component R.sub.dc and variable component R.sub.ac. R.sub.ac represents the change in resistance due to the fluctuating magnetic field. In order to detect variations in R.sub.MR, a bias current is passed through the sensor stripe of the MR head. This bias current serves to convert variations in stripe resistance produced by the magnetic field into a voltage signal.
Existing MR head systems commonly bias the MR head with either a constant bias current or (to a lesser extent) a constant voltage. A differential voltage amplifier is commonly used to amplify the ac component V.sub.ac of the voltage signal across the MR head. Other detection methods involve measurement of the ac component I.sub.ac of the current signal through the MR head.
For constant current biasing, a constant current I.sub.BIAS is passed through the sensor stripe. For the voltage case, the ac voltage component is then V.sub.ac =I.sub.BIAS R.sub.ac.
There are a number of problems associated with constant current biasing. These problems result from sub-micron dimensional variations of the head, which commonly occur among a given population of MR heads. Such dimensional variations result from imperfections in the fabrication process.
One such problem is related to the design of the sense amplifier and automatic gain control (AGC) section of the MR head system. Dimensional variations affect both R.sub.ac and R.sub.dc of the MR head. Since V.sub.ac =I.sub.BIAS R.sub.ac, the amplitude of V.sub.ac is affected by these dimensional variations as well. Consequently, the amplifier/AGC section of an MR head system using a constant bias current must be designed for a voltage signal having a wide dynamic range.
A second problem is related to the current density in the MR head. Both the performance and the reliability of the MR head depend on the current density. When a constant bias current flows through the MR head, the current density depends on the cross-sectional area of the head. In order to maintain a high level of performance, it is desirable to maintain a high current density. A high current density results in a high read-back signal, and thus a high signal-to-noise ratio and a low error rate. However, if the current density is too high, the life of the MR head becomes limited as the result of electromigration, and the reliability rate decreases. Thus, there is a tradeoff between performance and reliability. Consequently, in order to achieve an acceptable balance between performance and reliability, it is important to maintain an "optimum" current density in the MR head.
Constant current biasing does not always permit the MR head to be biased with this optimum current density level. Cross-sectional area variations among a population of MR heads results in a wide distribution of current density variations. To achieve acceptable part reliability, a wide current density design tolerance must therefore be maintained, resulting in a lower mean current density, and lower performance.
The problems associated with constant current biasing discussed above can potentially be solved by biasing the MR head with a constant voltage, V.sub.BIAS. The ac component of the voltage across the MR head is then V.sub.ac =(R.sub.ac /R.sub.dc)V.sub.BIAS. Since R.sub.ac and R.sub.dc are affected equally by dimensional variations, the effect of these resistance variations cancel. Thus, V.sub.ac is not affected by dimensional variations of the MR head, and the amplifier/AGC section of the MR system can have a narrower dymanic range tolerance.
Constant voltage biasing additionally provides the advantage of maintaining a constant current density through the MR head, independent of cross-sectional area variations. Thus, MR head systems utilizing a constant voltage biasing scheme can be designed with a narrower current density tolerance, and a higher mean current density. Such designs can achieve a higher level of performance without a corresponding decrease in part reliability.
Although constant voltage biasing provides considerable advantages over constant current biasing, the ability to adequately provide a constant voltage bias across the MR head is prevented by the existence of a parasitic resistance R.sub.c in the conductors of cables connecting the biasing circuit to the MR head. This cable resistance typically includes: 1) MR head-to-bonding pad trace resistance, 2) bonding pad-to-flex circuit mag wire resistance, 3) flex-circuit to IC resistance, and 4) parasitic resistances in the integrated circuit voltage biasing source. R.sub.c typically has a value of about 5Ohms, in comparison to a typical R.sub.MR value of about 15 Ohms.
The addition of R.sub.C to the MR bias circuit creates a voltage divider circuit in which a portion of V.sub.BIAS falls across R.sub.c. This prevents the full bias voltage V.sub.BIAS from being applied to the MR head. Further, as R.sub.MR varies within a population of heads, the portion of V.sub.BIAS which is applied to the MR head still vary due to the voltage divider effects of R.sub.C. As a result, true constant voltage biasing is not achieved, and the advantages of constant voltage biasing are not fully realized.
What is needed is a means for providing a constant voltage bias to an MR head, wherein the effects of parasitic cable resistances are negated.